Hemostasis is the process of arresting the outflow of blood from an injured blood vessel. For mammals, as well as many other organisms, the hemostatic process is critically important for continued survival. Defects in the hemostatic process can result in, for example, the inability to effectively form blood clots that serve to stop the loss of blood following vascular injury. In humans, individuals who suffer from an inability to form blood clots are called hemophiliacs. Of particular concern for hemophiliacs is the life-threatening risk that once started, bleeding will never cease.
Generally, hemophiliacs lack the ability to produce effective amounts of one or more substances required to form blood clots. For example, hemophiliacs who suffer from hemophilia A (also called “classic hemophilia”) have an inability to produce effective levels of Factor VIII (also known as “antihemophilia factor A,” “antihemophilic globulin,” and “AHG”). Factor VIII is a key component of one of several “cascades” of reactions that result in the formation of blood clots. Critical for the cascade of reactions referred to as the “intrinsic pathway,” Factor VIII ultimately influences the conversion of fibrinogen into the major component of blood clots, fibrin.
Although the intrinsic pathway of blood clot formation is relatively complex, the role of Factor VIII can be described briefly. In the presence of relatively small amounts of thrombin (released, for example, by the cells of ruptured tissues), Factor VIII is converted into its activated form known as Factor VIIIa. Factor VIIIa (along with other substances), in turn, activates another factor, Factor X into Factor Xa. Thereafter, Factor Xa (along with other substances) converts prothrombin into thrombin, with the result that a relatively large amount of thrombin is produced over time. Relatively large amounts of thrombin effectively convert fibrinogen into fibrin. Fibrin, in turn, forms the matrix or lattice responsible for the formation of blood clots. Factor VIII's role in the intrinsic pathway of blood clotting is shown schematically below.

Affecting one or two males for every 10,000 live births in all populations, hemophilia A can result from any one of a variety of mutations of the Factor VIII gene, which is located on the X-chromosome. Depending on the particular mutation, hemophilia A can manifest itself as severe, moderate or mild. Individuals suffering from the severest forms of hemophilia A entirely lack the ability to express active forms of Factor VIII. Clinically, individuals affected with hemophilia A suffer from muscle hemorrhage, joint hemorrhage, easy bruising, and prolonged bleeding from wounds. The mean age of individuals suffering from hemophilia A without treatment is twenty. Current treatment of hemophilia A involves the infusion of exogenous Factor VIII concentrate collected from human plasma or prepared via recombinant DNA techniques. Because these treatments serve only to supplement the lack of effective levels of Factor VIII, individuals suffering from Factor VIII require regular injections of Factor VIII concentrate throughout their lives.
Several commercial forms of Factor VIII concentrates are available to provide replacement therapy for patients suffering from hemophilia A. For example, blood-derived Factor VIII concentrate products sold under the HEMOFIL® M (Baxter, Deerfield, Ill.), KOATE®-DVI (Bayer, Research Tringle Park, N.C.), MONARC-M™ (American Red Cross, Washington, D.C.), and MONOCLATE-P® (Aventis, Bridgewater, N.J.) brands. With respect to recombinantly prepared Factor VIII concentrates, commercial products are provided under the HELIXATE® FS (Aventis, Bridgewater, N.J.), KOGENATE FS® (Bayer, Research Triangle Park, N.C.), RECOMBINATE® (Baxter, Deerfield, Ill.), ADVATE® (Baxter, Deerfield, Ill.), and REFACTO® (Wyeth/Genetics Institute, Cambridge, Mass.) brands.
Generally, recombinant sources of Factor VIII concentrates are favored over blood-derived sources since the latter involves the risk of transmitting viruses and/or other diseases associated with blood donation. While recombinant-based formulations avoid these drawbacks, the processing of recombinant-based products often requires the presence of certain proteins such as albumin, which are inevitably present in the final formulation administered to the patient. Often, patients who receive such formulations develop allergic reactions to these foreign proteins. In any event, both blood-derived and recombinant-based products suffer from the disadvantage of repeated administration.
PEGylation, or the attachment of a poly(ethylene glycol) derivative to a protein, has been described as a means to reduce immunogenicity as well as a means to prolong a protein's in vivo half-life. With respect to Factor VIII, however, previous experiences with forming protein-polymer conjugates has proven to be of little predictive value vis-à-vis polymer coupling to Factor VIII. See U.S. Pat. No. 4,970,300.
Notwithstanding these difficulties, attempts of preparing satisfactory compositions of conjugates of certain polymers to Factor VIII have been described. For example, previously referenced U.S. Pat. No. 4,970,300 describes the PEGylation of Factor VIII using a specific poly(ethylene glycol) derivative having a molecular weight within the range of about 500 to 5,000. In addition, U.S. Pat. No. 6,048,720 describes efficient protection against degradation in an in vitro environment when four to five monomethoxy poly(ethylene glycol) strands are conjugated to Factor VIII.
None of these described conjugates, however, has proven to satisfactorily address the problems associated with current Factor VIII-based therapies. For example, conjugates comprised of relatively small polymers (e.g., of about 5,000 Daltons or less) may not suitably provide extended in vivo half-life and/or sufficiently reduced immune response. In addition, conjugates having many individual polymers attached to Factor VIII are more likely to have reduced activity as a result of the polymer(s) blocking sites necessary for activity.
Thus, there remains a need in the art to provide additional conjugates between water-soluble polymers and moieties having Factor VIII activity. The present invention is therefore directed to such conjugates as well as compositions comprising the conjugates and related methods as described herein, which are believed to be new and completely unsuggested by the art.